In order to subject an object, such as a future integrated circuit, to a photolithography operation, chosen zones of the object, precoated with a photoresist, are exposed to a source of radiation in the visible or in the ultraviolet. This radiation exposes the aforementioned zones and consequently results in local etched features in the object.
As a general rule, the etched features are finer the shorter the wavelength of the radiation. Document EP-1 222 842 has proposed a source of radiation in the extreme ultraviolet (called EUV hereafter) and its application to photolithography. The wavelength of the radiation extends from about 8 nanometers to about 25 nanometers, making it possible to achieve etched features with a fineness of typically less than around 100 nanometers. The radiation emanates from a plasma, which is the site of interaction between a laser beam and a mist comprising micron-sized xenon and/or water droplets. The laser source may be in the form of a nanosecond laser of Nd:YAG type. It excites a jet of particles output by a nozzle, thus forming the aforementioned mist of droplets.
Also known, from the publication WO 02/32197, is extreme ultraviolet radiation resulting from the excitation of a jet of liquid xenon.
In a more recent development, described in publication FR-2 814 599, EUV radiation is obtained by the interaction between several laser beams and a jet of particles, such as a xenon mist. In particular, laser sources are designed to emit shots substantially in one and the same region of the jet and substantially at the same time. Thus, it will be understood that, by combining several laser sources that irradiate the jet of particles substantially at the same time, the peak power of the radiation that produces the plasma is increased. The frequency of the laser shots is of the order of one to a few tens of kHz. Thus, the expression “substantially at the same time” means that, at each firing period, for example every 0.1 ms, a certain number of individual light pulses, each generated by an individual laser, are grouped together into a collection of pulses that are simultaneous and/or juxtaposed over time, these being called composite pulses. Optionally, this juxtaposition may constitute two groups of pulses and two respective instants, namely a first group for striking the plasma and a second group for increasing it, the time shift between these two groups being much shorter than the shot repetition period. However, it should be noted that the shift of the individual pulses in space and time has the purpose of adapting the energy influx to the requirements of the plasma in accordance with its temporal evolution, so as to improve the energy balance. The device described does not in any way seek to make a fine adjustment of the light power delivered.
Since the publication of this document FR-2 814 599, the expectations of industrial companies demanding an etching process in the extreme ultraviolet have grown significantly. At the present time, these industrial companies require, in the manner of the Dutch consortium ASML:                a substantially continuous fabrication process, with a run speed of the semiconductor wafer to be irradiated of 400 mm/s;        an extreme ultraviolet pulse repetition frequency of 10 kHz;        at each point on the surface to be irradiated, a cumulative extreme ultraviolet does of 5 mJ/cm2, provided by a succession of 50 pulses; and        this received dose having an error distribution that has to be less than 0.1% of the setpoint.        
The last constraint mentioned represents already by itself a technological challenge with no solution in the prior art to the knowledge of the inventors. This is because certain phenomena associated with the generation of a plasma by laser illumination of a target, especially when the latter consists of xenon aggregates, are still poorly understood or, at the least, are the subject of much uncertainty. The position of the jet of particles and laser beams may be temporally shifted in terms of position, especially because of substantial temperature variations in the interaction chamber. The jet itself undergoes inevitable fluctuations.
Document U.S. Pat. No. 4,804,978 describes a way of controlling an energy dose for photolithography using attenuator filters mounted on a motorized wheel. However, this solution does not allow operation at a high rate since the laser shots are interrupted while a filter is put into place. Moreover, a continuous relative displacement of the object to be etched with respect to the source is incompatible with this process, which on the contrary requires complete immobility until the cumulative energy dose has been obtained. Finally, the existence of a finite number of filters, corresponding to discrete attenuation values, does not allow the precise cumulative energy dose to be provided.
Document U.S. Pat. No. 6,034,978 describes another way of controlling the stability of the radiation source between two pulses so as to ensure stability of the energy dose delivered. In particular, said document provides a way of controlling the temperature of the gaseous medium, which is the source of the radiation, so as to control the intensity stability of the radiation. Now, this solution cannot be transposed to the device described in FR-2 814 599 since the radiation source is not a plasma, as in FR-2 814 599, but rather a gas laser. Such a system for regulating the emission, by cooling the gas laser by controlled circulation of water, is not easily applicable in the device of FR-2 814 599 with a jet of particles in the form of a mist. Firstly, such a process cannot take into account the fluctuations in the efficiency of conversion between laser energy and extreme ultraviolet energy, thereby making it impossible to apply it to the generation of extreme ultraviolet radiation by interaction of coherent light on a plasma-generating target. Secondly, in the prior art it does not seem that such a laser is capable of delivering intense pulsed energy at a high rate specified by the ASML consortium.
In conclusion, to the knowledge of the inventors, the prior art neither offers nor suggests any other method or device allowing extreme ultraviolet photoetching which, on the one hand, is effectively continuous, that is to say without steps of the method, other than the displacement of the object, slowing down the rate of the extreme ultraviolet pulses, and which, on the other hand, provides a standard deviation in the distribution of the error in the doses received of the order of 0.1% or less. The aim of the present invention is to satisfy this lack, and it describes a method allowing extreme ultraviolet photoetching, which, on the one hand, is effectively continuous, that is to say without steps of the process other than the displacement of the object slowing down the rate of the extreme ultraviolet pulses and which, on the other hand, provides a standard deviation in the distribution of the error in the doses received of the order of 0.1% or less.
It is another object of the present invention to provide a device for applying this process.